Switches of this type are usually disposed in series with a power switch in the load circuit so that their own contact-closing and contact-opening operations occur under virtual open-circuit conditions. A group of such switches may be used, for example, to connect the open power switch to any one of several associated supply lines in order to establish a selected current path for a given load. These switches also serve to indicate, by their readily observable position, whether or not a particular circuit is connected to the power line or disconnected therefrom.
Even though a switch of the kind here considered opens and closes in the absence of load current, it may still be traversed by residual commutation or capacitive currents at relatively low voltage giving rise to unavoidable arcing on the opening stroke. When closed, the switch must be able to carry not only the normal load current but also, in the event of a short circuit, a greatly increased current which flows until the associated power switch is opened. The switch, of course, should always operate with a minimum contact resistance.
Since silver is a highly conductive metal, its utilization on a contact surface of a switch is well known in the art. Frequent arcing, however, is known to damage such contact surfaces as more fully discussed hereinafter.
When the disconnect switch comprises two parallelogrammatic linkages in adjoining planes forming scissor arms whose extremities constitute pairs of movable contactors designed to grip a common elongate countercontact, the engagement of the two pairs of contactors with the countercontact should be as uniform as possible. In operation, however, the load current passing in parallel through the two linkages generates an electromagnetic field subjecting these linkages to mechanical stresses and warping which may impair the engagement of their contactors with the countercontact. If the contact surfaces become temporarily separated as a result of these stresses, they can be greatly harmed by the resulting arcing under load.